Tracheobronchial epithelial adherence studies using H. influenzae, P. aeruginosa, and S. pneumoniae were performed with airway cells obtained from two normal non-smokers (I), eight smokers without chronic bronchitis (II), and seven smokers with a history of chronic bronchitis (III). Two subjects from Group III had recent exacerbations of bronchitis prior to bronchoscopy. Among the 15 humans studied without active bronchitis (Groups I, II, III), no bacterial adherence of the above test organisms to tracheobronchial cells, individually or in clusters, was observed in vitro and no bacteria were attached to these cells in vivo. None of these subjects demonstrated lower respiratory tract bacterial colonization, as determined by the protected sterile sleeve catheter technique. In contrast, among the two patients with acute exacerbations of bronchitis, H. influenzae was isolated in one case and E. coli with anaerobic streptococcus in the second subject. In these studies, it was not possible to accurately quantitate tracheobronchial adherence in vitro in the presence of acute inflammation. However, occasional cells were observed with large numbers of adherent bacteria. These preliminary studies in humans indicate that distal airways contain no cultivatible bacteria and no cell-adherent bacteria in the absence of inflammation while both are present during episodes of acute bronchitis.